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Ukraine’s Prosecutor General, Iryna Venediktova, said the bodies of 410 civilians had been removed from Kyiv-area towns that were recently retaken from Russian troops.
Ms Venediktova posted on Facebook that the bodies were recovered on Friday, Saturday and Sunday. She said 140 of them had undergone examination by prosecutors and other specialists.
Ukrainian Deputy Prime Minister Iryna Vereshchuk said the mayor of the village of Motyzhyn in the Kyiv region was murdered while being held by Russian troops.
Ms Vereshchuk said 11 mayors and community heads were being held by the Russians across Ukraine.
In Bucha, mines have been left scattered around homes and corpses.
The body of a missing Ukrainian photographer, Maksim Levin, was found in a nearby village.
A mass grave in the grounds of a church was still open, with hands and feet poking through the red clay heaped on top, Reuters reported.
Residents of the city have said the people were killed by Russian soldiers without any apparent provocation, corroborating mounting evidence of alleged war crimes.
"These people were just walking and they [the Russian soldiers] shot them without any reason. Bang,” one Bucha resident told AP.
"In the next neighbourhood, Stekolka, it was even worse. They would shoot without asking any questions."
In a video address on Sunday, Ukrainian President Volodymyr Zelenskyy denounced targeted killings of civilians in towns that the Russians occupied.
Mr Zelenskyy called the killers “freaks who do not know how to do otherwise".
He said that more atrocities could be revealed if Russian forces were driven out of other occupied areas.
International leaders have condemned the attacks in the Kyiv-area towns after harrowing accounts from civilians and graphic images of bodies with hands tied behind their backs.
Russia’s Defence Ministry has rejected the claims of atrocities against civilians in Bucha and other suburbs of Kyiv.
UK Prime Minister Boris Johnson has condemned Russia’s “despicable attacks” against Ukrainian civilians in Irpin and Bucha, adding that “we will not rest until justice is served”.
Mr Johnson said “no denial or disinformation from the Kremlin” can conceal that Russian President Vladimir Putin is “desperate” and “his invasion is failing”.
He said the UK was increasing its sanctions and military support, and “bolstering” humanitarian help for those on the ground.
“The UK has been at the forefront of supporting the International Criminal Court’s investigation into atrocities committed in Ukraine,” Mr Johnson said.
“The Justice Secretary has authorised additional financial support and the deployment of specialist investigators. We will not rest until justice is served.”
The Ministry of Justice said on March 24 that £1 million ($1.3m) in funding would be provided, and soldiers with military expertise would be assigned to the ICC to help uncover evidence.
New Scotland Yard’s War Crimes Team would also assist the investigation, the ministry said.
Britain’s Foreign Secretary Liz Truss said she was “appalled by atrocities in Bucha and other towns in Ukraine”.
Ms Truss said the UK would fully support any move by the International Criminal Court to investigate Russia for war crimes.
"As Russian troops are forced into retreat, we are seeing increasing evidence of appalling acts by the invading forces in towns such as Irpin and Bucha," she said, referring to places near Kyiv.
"Their indiscriminate attacks against innocent civilians during Russia's illegal and unjustified invasion of Ukraine must be investigated as war crimes."
Russia has previously denied attacking civilians and rejected allegations of war crimes in what it calls a "special military operation" in Ukraine.
Ukraine’s attorney general is gathering a dossier of claims about the Russian use of local children to avoid fire when in retreat from around Ukraine’s capital and elsewhere.
The horrifying witness accounts and physical evidence have come to light after the pull-out of Russian troops from areas around Kyiv after more than five weeks of fighting.
Ukrainian soldiers have reportedly regained control of more than 30 towns and villages in the area.
Russia continued withdrawing some of its ground forces from areas around Kyiv after saying this week that it would reduce military activity near the Ukrainian capital and the northern city of Chernihiv.